1. Field of the Invention
The present invention relates to a process for the upgrading of hydrocarbon mixtures which boil within the naphtha range containing sulfur impurities, i.e. a hydrodesulfuration process with contemporaneous skeleton isomerization and reduced hydrogenation degree of the olefins contained in said hydrocarbon mixtures, the whole process being carried out in a single step. The process is effected in the presence of a catalytic system comprising a metal of group VI B, a metal of group VIII and a carrier of acid nature consisting of a mesoporous silico-alumina.
2. Description of the Background
This catalytic system can be used, in particular, for the upgrading of mixtures of hydrocarbons which boil within the naphtha range deriving from cracking processes, preferably mixtures of hydrocarbons having a boiling point within the naphtha range deriving from FCC catalytic cracking (Fluid Catalytic Cracking).
Hydrocarbons which boil within the naphtha range deriving from FCC (i.e. gasoline cut) are used as blending component of gasolines. For this purpose, it is necessary for them to have a high octane number together with a low sulfur content, to conform with the law restrictions which are becoming increasingly more severe, in order to reduce the emission of contaminants. The sulfur present in gasoline mixtures in fact mainly comes (>90%) from the gasoline cut deriving from FCC.
This cut is also rich in olefins which have a high octane number. Hydrogenation processes used for desulfuration also hydrogenate the olefins present with a consequent considerable reduction in the octane number (RON and MON). The necessity has therefore been felt for finding a catalytic system which decreases the sulfur content in hydrocarbon mixtures which boil within the naphtha range and, at the same time, minimizes the octane loss (RON and MON), which can be achieved, for example, by the skeleton isomerization of the olefins present and/or by inhibiting the hydrogenation of the olefinic double bond.
The use of zeolites with a medium pore dimension as isomerization catalysts and the consequent recovery of octane in the charges already subjected to desulfuration are already known (U.S. Pat. No. 5,298,150, U.S. Pat. No. 5,320,742, U.S. Pat. No. 5,326,462, U.S. Pat. No. 5,318,690, U.S. Pat. No 5,360,532, U.S. Pat. No. 5,500,108, U.S. Pat. No. 5,510,016, U.S. Pat. No. 5,554,274, U.S. Pat. No. 5,994,39). In these known processes, in order to obtain hydrodesulfuration with a reduced octane loss, it is necessary to operate in two steps, using in the first step catalysts suitable for desulfuration and in the second step catalysts for recovering the octane number.
U.S. Pat. No. 5,378,352 describes a process in a single step for desulfurating hydrocarbon fractions, with boiling points within the range of gasolines, using a catalyst which comprises a metal of group VIII, a metal of group VI B and a zeolite.
MI97A. 002288 describes a catalytic system capable of isomerizing n-paraffins having a number of carbon atoms higher than 15 which comprises:    a) a carrier of acid nature consisting of a silica and alumina gel, amorphous to X-rays, with a molar ratio SiO2/Al2O3 ranging from 30/1 to 500/1, having a surface area of 500 to 1000 m2/g, a porosity ranging from 0.3 to 0.6 ml/g and a pore diameter within the range of 10–40 Å;    b) a mixture of metals belonging to groups VI B and VIII deposited on the carrier in an overall quantity of 2 to 50% by weight with respect to the total of (a)+(b).
The acid carrier of the catalyst preferably has a ratio SiO2/Al2O3 ranging from 50/1 to 300/1 and a porosity of 0.4 to 0.5 ml/g.
The mixture of metals (b) preferably consists of a metal of group VI B selected from molybdenum and tungsten, in a quantity ranging from 5 to 35% by weight, and a non-noble metal of group VIII selected from nickel and cobalt, in a quantity ranging from 0.1 to 5% by weight.
The carrier based on silica and alumina gel can be conveniently prepared according to what is described in the patent U.S. Pat. No. 5,049,536 or in patent application EP 659478. This carrier can be used as such for the deposition of the metal phase (b) or in extruded form, as described for example in EP 550922 and EP 665055. The metal phase (b) of the catalyst can be introduced by means of aqueous impregnation by wetting the carrier, or extruded carrier, with an aqueous solution of a compound of a metal of group VI B, and drying the resulting product which is then impregnated with an aqueous solution of a compound of the metal of group VIII and dried again. Calcination is then carried out in an oxidating atmosphere at a temperature ranging from 200 to 600° C. Alternatively, a single aqueous solution containing both the compounds of the metals of groups VI B and VIII can be used for contemporaneously introducing these metals. In the case of alcohol impregnation, the acid carrier is suspended in an alcohol solution containing both compounds of the metals of groups VI B and VIII. After impregnation, the solid is dried and calcined.
These catalytic compositions are useful in the hydroisomerization of n-paraffins having a number of carbon atoms higher than 15 to obtain bases for lubricating oils characterized by a low “pour point”, and a high viscosity index.